Question: The scale on a map is 1cm : 9km. If two cities are 4cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 1cm on the map corresponds to an actual distance of 9km. A distance of 4cm on the map is the same as 4 $\cdot$ 1cm. The actual distance between the two cities is 4 $\cdot$ 9km, or 36km.